This invention relates to an achromatic objective lens having an aperture ratio of about 1:12 and a viewing angle of about .+-.1.degree. for use as an objective lens of an astronomical telescope. In an astronomical telescope or the like having a long focal length and a narrow view angle, it is most important to eliminate the secondary spectrum of chromatic aberration and to compensate for spherical aberration and comatic aberration, in various aberration compensations. Such aberrations greatly effect the sharpness of the focused image.
In order to eliminate or compensate for the particular aberrations, for example, the secondary spectrum, fluorite having a particularly low dispersion or similar optical material has been heretofore used. However, such a material is expensive and cannot be readily mass-produced. For this reason, a so-called apochromatic lens composed of three lenses not using any particular glass material has been considered. The apochromatic must have lens surfaces having large curvatures due to the strong lens power of each lens. This results in difficulty of manufacturing and at the same time, the resulting lens has a large amount of the residual and the chromatic aberrations due to the spherical aberration. Therefore, the sharpness of the image is low and the aperture ratio is limited to about 1:15.